24. In the atmospheric engine as first designed, the slow process of condensation by the application of the condensing water to the exterior of the cylinder to produce the vacuum caused the strokes of the engine to take place at very long intervals. An improvement was, however, soon effected, which immensely increased this rapidity of condensation. A jet of water was thrown directly into the cylinder, thus effecting for the Newcomen engine just what Desaguliers had previously done for the Savery engine. As thus improved, the Newcomen engine is shown in Fig. 11.
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| Fig. 11.—Newcomen's Engine, a. d. 1705. |
Here d is the boiler. Steam passes from it through the cock d, and up into the cylinder a, equilibrating the pressure of the atmosphere, and allowing the heavy pump-rod k to fall, and, by its greater weight, acting through the beam i i, to raise the piston s to the position shown.
The cock d being shut, f is then opened, and a jet of water from the reservoir g enters the cylinder, producing a vacuum by the condensation of the steam. The pressure of the air above the piston now forces it down, again raising the pump-rods, and thus the engine works on indefinitely.
The pipe h is used for the purpose of keeping the upper side of the piston covered with water, to prevent air-leaks—a device of Newcomen.
Two gauge-cocks, c, c, and a safety-valve, N, are represented in the figure, but it will be noticed that the latter is quite different from the now usual form. Here, the pressure used was hardly greater than that of the atmosphere, and the weight of the valve itself was ordinarily sufficient to keep it down. The rod m was intended to carry a counter-weight when needed.
The condensing water, together with the water of condensation, flows off through the open pipe p. Newcomen's first engine made six or eight strokes a minute; the later and improved engines made ten or twelve.
25. The steam-engine has now assumed a form that somewhat resembles the modern machine.
An important defect still existed in the necessity of keeping an attendant by the engine to open and shut the cocks. A bright boy, however, Humphrey Potter, to whom was assigned this duty on a Newcomen engine in 1713, contrived what he called a scoggan—a
